1. Field of the Invention
The present invention relates to a high-pressure discharge lamp employing an outer bulb, which is provided with a lamp cap and surrounds a discharge vessel having a ceramic wall with a clearance, an electric conductor extending in the clear space over a length L between the lamp cap and an electrode of the discharge vessel, and the conductor being predominantly made of stainless steel.
2. Description of the Related Art
A lamp known in the prior art is disclosed in U.S. Pat. No. 4,758,759. Such a lamp, embodied so as to be a high-pressure sodium lamp, is used, for example, on a large scale for public lighting applications. In an embodiment wherein the lamp comprises metal halide, it is particularly suitable as a light source for interior lighting. Ceramic wall is to be taken to mean in this description and the claims, both a wall of a gastight light-transmitting metal oxide, such as sapphire or densely sintered polycrystalline Al2O3, as well as a wall of metal nitride, such as AlN. Stainless steel is to be taken to mean in this description and the claims, both Cr-alloyed ferritic steel and Cr and Ni-alloyed austenitic steel which is resistant to temperatures of 500xc2x0 C. and higher.
The discharge vessel of the known lamp employs two inner electrodes between which a discharge takes place during operation of the lamp. Each electrode is electrically connected to the lamp cap by a conductor. The conductors generally also form a part of the suspension of the discharge vessel. Suitable constructions enable Ni-plated iron to be used. However, this has the drawback that the necessary constructions are fairly complex and heavy. Another drawback resides in that Ni-plated iron has a small residual strength as a result of which it proves unsuitable for use in simpler and lighter constructions wherein the conductor is embodied so as to be self-supporting. The aforementioned drawbacks can be obviated by using stainless steel. However, in practice, stainless steel gives rise to blackening of the outer bulb, which is a disadvantage.
It is an object of the invention to provide a high pressure discharge lamp that obviates the aforementioned drawbacks with the prior art. To achieve this, a high pressure discharge lamp in accordance with the invention comprises a conductor with an Ni coating over at least a part Ld of a length L of the conductor.
It has been found that said Ni coating is a suitable for counteracting blackening and, on the other hand, it enables the conductor to be readily weldable. The latter is in sharp contrast with, for example, a coating of Cr oxide, known per se, which leads to a poor weldability of the metal conductor. In addition, the high temperature generally required for applying such a coating readily leads to an undesirable reduction of the residual strength of the conductor in question.
Preferably, the part Ld of the conductor where the coating is present, extends along the discharge vessel. The part Ld is the portion of the conductor that reaches a maximum temperature during operation of the lamp.
In a further, advantageous embodiment of the lamp in accordance with the invention, the conductor is made from ferritic stainless steel. An important advantage of ferritic steel is that only a comparatively small temperature increase is required to apply the Ni coating, as a result of which the ferritic steel preserves sufficient residual strength and hence largely preserves its self-supporting capacity. In addition, the nickel-coated ferritic stainless steel can be readily welded. Preferably, the Ni coating forms a layer that encloses the conductor. With a view to a reproducible, industrial-scale manufacture of the lamp in accordance with the invention, the layer advantageously has a thickness of at least 3 xcexcm.